1. Field of the Invention
The invention relates to a device for insulating and sealing electrode holders in a reactor for depositing polycrystalline silicon and to a process for producing polycrystalline silicon using such a device.
2. Description of the Related Art
High-purity silicon is generally produced by the Siemens process. This comprises introducing a reaction gas comprising hydrogen and one or more silicon-containing components into the reactor fitted with support bodies heated by direct passage of current upon which solid silicon is deposited. Preferably employed silicon-containing compounds are silane (SiH4), monochlorosilane (SiH3Cl), dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3), tetrachlorosilane (SiCl4) and mixtures thereof.
Each support body is generally composed of two thin filament rods and one bridge which generally connects adjacent rods at their free ends. The filament rods are most commonly fabricated from mono- or polycrystalline silicon, metals/alloys or carbon being employed more rarely. The filament rods are slotted vertically into electrodes disposed on the reactor floor which provide the connection to the electrode holder and current supply. High-purity polysilicon is deposited on the heated filament rods and the horizontal bridge to increase the diameter thereof over time. The process is terminated once the desired diameter has been achieved.
The silicon rods are held in the CVD reactor by special electrodes generally made of graphite. In each case two filament rods having different voltage polarities at the electrode holders are connected by a bridge at the other slim rod end to form a closed electrical circuit. Electrical energy for heating the slim rods is supplied via the electrodes and their electrode holders. This causes the diameter of the slim rods to increase. The electrode simultaneously grows into the rod base of the silicon rods, starting at its tip. Once a desired target diameter for the silicon rods has been achieved the deposition process is terminated and the silicon rods are cooled and removed.
The sealing of the electrode holder which passes through the floor plate is of particular importance. The use of electrode sealing bodies has been proposed to this end, importance attaching in particular to the arrangement and shape of the electrode sealing body and the material employed.
Disposed between the top of the electrode holder, which protrudes into the deposition equipment, and the floor plate is an annular body. this body typically has two functions: 1) sealing of the electrode holder feedthrough, and 2) electrical insulation of the electrode holder from the floor plate.
The high gas-space temperature in the CVD reactor necessitates thermal protection of a hydrocarbon-based sealing body. Insufficient thermal protection results in premature wear of the sealing bodies due to scorching of the sealing bodies, thermally induced flow of the sealing body, reactor leaks, the distance between electrode holder and floor plate falling below the minimum value, and ground faults at charred sealing bodies. Ground faults or leaks result in outage of the deposition equipment and hence in the deposition process being aborted. This results in a lower yield and higher costs.
US 20110305604 A1 discloses shielding the electrode seals from thermal stress using protective rings made of quartz. The reactor floor has a special configuration. The reactor floor comprises a first region and a second region. The first region is formed by a plate facing toward the interior of the reactor and an intermediate plate carrying the nozzles. The second region of the reactor floor is formed by the intermediate plate and a floor plate carrying the supply connections for the filaments. The cooling water is fed into the first region thus formed in order thus to cool the reactor bottom. The filaments themselves are seated in a graphite adapter. This graphite adapter engages with a graphite clamping ring, which itself interacts with the plate via a quartz ring. The cooling water connections for the filaments may be in the form of quick-fit couplings.
WO 2011116990 A1 describes an electrode holder having a quartz cover ring. The process chamber unit is composed of a contacting and clamping unit, a base element, a quartz covering disk, and a quartz covering ring. The contact and clamping unit is composed of a plurality of contacting elements which can be moved relative to one another and form a receiving space for a silicon slim rod. The contacting and clamping unit may be introduced into a corresponding receiving space of the base element, the receiving space for the silicon slim rod narrowing on introduction into the base element so that said slim rod is thus securely clamped and electrically contacted. The base element also comprises a lower receiving space for receiving a contacting tip of the feedthrough unit. The quartz covering disk has central openings for feeding through the contacting tip of the feedthrough unit. The quartz covering ring has dimensions such that it can at least partially radially surround a region of the feedthrough unit disposed inside a process chamber of a CVD-reactor.
However, since the quartz, used as described in the relevant art, has a low thermal conductivity, these components become so hot under deposition conditions that a thin silicon layer grows on their surface at high temperature. The silicon layer is electrically conducting under these conditions which leads to a ground fault.
WO 2011092276 A1 describes an electrode holder where the sealing element between the electrode holder and the floor plate is protected against the effects of temperature by a circumferential ceramic ring. A plurality of electrodes are secured in a floor of the reactor. These electrodes carry filament rods seated in an electrode body which supplies current to the electrodes/filament rods. The electrode body itself is mechanically prestressed in the direction of the top face of the floor of the reactor by a plurality of resilient elements. A radially circumferential sealing element is inserted between the top face of the floor of the reactor and a ring of the electrode body which is parallel to the top face of the floor. The sealing element itself is shielded by a ceramic ring in the region between the top face of the floor of the reactor and the ring of the electrode body which is parallel thereto.
The sealing element is made of PTFE and assumes both the sealing function and the insulating function. The ceramic ring serves as a heat shield for the sealing ring. However, subjecting PTFE to thermal stress above 250° C. results in scorching/cracking at the seal surface and flow of the sealing body. The distance between the top of the electrode holder and the floor plate thus falls below a minimum value leading to electrical arcing/ground faults from the electrode holder to the floor plate. The scorching/cracking also releases carbon compounds which lead to contamination of the silicon rods to be deposited due to incorporation of carbon.
US 20130011581 A1 discloses a device for protecting electrode holders in CVD reactors which comprises an electrode which is suitable for accommodating a filament rod and is disposed on an electrode holder made of an electrically conductive material and mounted in a recess in a floor plate, wherein an intermediate space between the electrode holder and the floor plate is sealed with a sealing material and the sealing material is protected by a protective body constructed from one or more parts and arranged in a ring shape around the electrodes, wherein the height of the protective body increases at least in sections in the direction of the electrode holder. This document provides for geometrical bodies arranged concentrically around the electrode holder, their height decreasing with an increasing distance from the electrode holder. The body may also be composed of one part. This provides for thermal protection for the sealing and insulating body of the electrode holder and also for flow modification at the rod base of the deposited polysilicon rods which has a positive influence on the fallover rate.
The devices according to WO 2011092276 A1 and according to US 20130011581 A1 can suffer from ground faults between the electrode holder and the floor plate due to silicon slivers which, on account of thermal stresses due to the high feed rate, spall off the silicon rods, fall between the electrode holder and the ceramic ring/support body and there produce an electrically conducting connection between the electrode holder and the floor plate. Short circuits entail abrupt process termination due to outage of the current supply for heating the rods. The rods cannot be deposited up to the intended end diameter. Thinner rods lead to lower plant capacity which results in considerable costs.
CN 202193621 U discloses a device providing two ceramic rings between the top of the electrode holder and the floor plate with a graphite gasket disposed between them.
However, this device provides no sealing function between the ceramic ring and the top of the electrode holder nor between the ceramic ring and the floor plate. The reactor consequently suffers from leaks.
CN 101565184 A discloses an insulating ring made of zirconium oxide ceramic material (ZrO2) between the top of the electrode holder and the floor plate. The insulating ring is recessed in the floor plate. An additional quartz ring is therefore required for insulation between the top of the electrode holder and the floor plate. Sealing is achieved via two graphite gaskets between the top of the electrode holder and the insulating ring and between the floor plate and the insulating ring. An O-ring is employed at the electrode feedthrough below the floor plate as a further seal.
CN 102616783 A discloses an insulating ring made of ceramic material between the top of the electrode holder and the floor plate. Sealing is achieved via two metal framed graphite gaskets above and below the insulating ring toward the top of the electrode holder and toward the floor plate respectively.
The problem with the latter two documents is that the graphite gasket requires high contact pressures to achieve sealing. Since ceramics material is brittle and has a low flexural strength, the sealing surfaces of the floor plate and the top of the electrode holder are subject to strict evenness requirements. Even the slightest unevenness, which is almost unavoidable in practice, results in fracture of the ceramic rings due to the high contact pressures. The reactor consequently suffers leaks.
WO 2014/143910 A1 discloses a sealing ring between the floor plate and the electrode holder comprising a base body made of a ceramic material with an upper and a lower groove, wherein sealing elements are inserted into the respective grooves. However it has become apparent that the sealing elements inserted into the grooves in the ceramic ring are subjected to a high level of thermal stress. Dynamic temperature changes at the sealing elements may lead to movement at the sealing elements caused by thermal expansion/contraction of the electrode holder, floor plate and seal. This can damage the surfaces of the sealing elements which may lead to leaks at the seals. This makes frequent seal replacement necessary resulting in reduced reactor service time.
US 2010058988 A1 provides for securing the electrode holder in the floor plate via a conical PTFE sealing and insulating element. The top face of the conical PTFE sealing element is compressed against the electrode holder via a flange (cross-sectional widening). An O-ring is additionally provided both between the sealing element and the electrode feedthrough through the floor plate and between the sealing element and the shaft of the electrode holder.
The compression of the conical sealing element impedes removal of the electrode holder. Flow of the PTFE sealing body can result in the distance between the electrode holder and the floor plate falling below the minimum value. This results in electrical arcing/ground faults.